1. Field of the Invention
The present invention relates to an optical system, an image projection apparatus including the same, and an image pickup device. In particular, the present invention relates to an optical system which corrects color aberration and a focus shift due to temperature variation, and an image projection apparatus including the same.
2. Description of Related Art
Recent years, an optical system which is used for an image pickup device of a TV camera, a digital camera and the like is required to have a compact size of an entire optical system and high optical performance. Among optical performances, if color aberration increases, image quality of a color image is extremely deteriorated.
Therefore, in order to correct color aberration appropriately, particularly to correct color aberration appropriately over the entire magnification range from the wide angle end to the telephoto end of the zoom lens, the secondary spectrum in the color aberration is corrected (see JP-A-8-297244).
In other words, the above-mentioned zoom lens includes a focus lens unit which performs focusing, a variator lens group which moves along the optical axis during zooming, and a compensator lens group which moves along the optical axis so as to compensate for image plane variation due to zooming. Three positive lenses of the compensator lens group are made of glass having low dispersion and anomalous dispersion characteristic, and one negative lens of the compensator lens group is made of high dispersion glass having a relatively small refractive index variation with respect to temperature variation.
In this way, by using optical glass having anomalous dispersion characteristic for three positive lenses, color aberration at the F-line and the C-line as well as secondary spectrum from the wide angle end to the telephoto end is reduced, so that variation amount of the axial color aberration is suppressed to be small.
In addition, usually, a zoom lens is constituted so as not to move the focus position during zooming. However, if temperature variation occurs, a refractive index of a lens varies, so that a focal length of the compensator lens group varies. Therefore, the focus position varies during zooming from the wide angle end to the telephoto end. In the zoom lens described above, one negative lens is made of optical glass having a relatively small refractive index variation with respect to temperature so as to reduce move amount of the focus position due to the zooming.
In the zoom lens described above, the color aberration is corrected, but the focus shift remains. In order to eliminate this focus shift, manual focusing operation or a mechanical member for automatic focus is necessary in accordance with zooming.
As a method of correcting such focus shift due to temperature variation, there is known a method of using thermal expansion of a lens barrel which holds the lenses.
For instance, an imaging lens including a plastic lens causes a focus shift due to temperature variation. Usually, a plastic lens has large refractive index variation due to temperature variation and a negative value of the refractive index variation. Therefore, when temperature of the imaging lens rises, a focus position of the imaging lens moves so that an image cannot be formed on a normal image plane of the image pickup element or the like. Therefore, a lens holding barrel which connects the plastic lens with the image pickup element is made of material having a coefficient of linear expansion corresponding to the movement of the focus position (U.S. Pat. No. 6,710,932). With this structure, when temperature rises, the lens holding barrel expands in the optical axis direction so as to cancel the focus position shift due to the refractive index variation of the lens.
However, in the above-mentioned structure, for downsizing the apparatus, the lens holding barrel may not have an enough length in the optical axis direction for obtaining a desired expansion. In addition, for a strength structure of the apparatus, material having an appropriate coefficient of linear expansion may not be selected for the lens holding barrel. Therefore, there is a limitation in the range in which the focus shift can be corrected. In this way, there is a problem in the above-mentioned structure that it is difficult to correct the focus shift appropriately at any time.
In addition, in the optical system that is used for an image projection apparatus such as a projector, three color images are combined, and the combined image is projected onto the projection surface so that the three color images are overlaid on the projection surface. Therefore, it is necessary to correct color shifts of the optical system appropriately with respect to the three color wavelengths. Further, since a high power lamp is used as a light source in the image projection apparatus, the optical system becomes very high temperature. Therefore, it is necessary that the optical system used in the image projection apparatus does not generate a focus shift with respect to temperature variation.
Therefore, there is proposed an image projection lens device including a first lens unit, a second lens unit, and a plurality of lens units disposed closer to the contraction side than the second lens unit, which are arranged in this order from the enlargement side (see U.S. Pat. No. 7,545,578). In this image projection lens device, a positive lens disposed closer to the contraction side than the second lens unit satisfies a predetermined condition concerning a focal length and a rate of change of the refractive index with respect to temperature. Further, a positive lens disposed closer to the contraction side than the first lens unit satisfies a predetermined condition concerning a focal length and an anomalous dispersion. When the condition is satisfied, the color aberration is corrected, and focus position variation due to temperature variation is suppressed.
However, although the above-mentioned structure suppresses focus position variation due to temperature variation, it is difficult to correct color aberration appropriately because only one type of anomalous dispersion glass is used.